<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>天气案例</title>
    <script type="text/javascript" src="../js/vue.js"></script>
</head>
<body>
    <!--
        监视属性watch:
         1. 当被监视的属性变化时，回调函数自动调用，进行相关操作
         2. 监视属性必须存在，才能进行监视！！
         3. 监视的两种写法：
            1. new Vue时传入watch配置
            2. 通过vm.$watch监视
    -->
    <div id="root">
        <h2>今天天气很{{info}}</h2>
        <button @click="changeWeather">切换天气</button>
        <hr>
        <h3>a的值是：{{number.a}}</h3>
        <button @click="numbers.a++">点我让a+1</button>
        <h3>a的值是：{{number.b}}</h3>
        <button @click="numbers.b++">点我让b+1</button>
        <button @click="numbers = {a:666, b: 888}">彻底替换掉numbers</button>
    </div>
</body>
<script type="text/javascript">
    new Vue({
        el: '#root',
        data: {
            isHot: true,
            numbers: {
                a:1,
                b:1
            }
        },
        computed: {
            info() {
                return this.isHot ? '炎热' : '凉爽'
            }
        },
        methods: {
            changeWeather() {
                this.isHot = !this.isHot
            }
        },
        watch: {
            isHot: {
                immediate: true, // 初始化时让handler调用一下
                // handler什么时候调用？当isHot发生变化时
                handler(newValue, oldValue) {
                    console.log('isHot被修改了',newValue, oldValue)
                }
            },

        }
    })
    vm.$watch('isHot',{
        immediate: true, // 初始化时让handler调用一下
        handler(newValue,oldValue) {
            console.log('isHot被修改了',newValue,oldValue)
        }
    })
</script>
</html>